Nuclear fuel particles

ABSTRACT

Nuclear fuel particles having fissile or fertile central cores surrounded by a buffer layer of low density pyrolytic carbon. A shell of dense silicon carbide, zirconium carbide or niobium carbide surrounds the buffer layer, and a layer of isotropic pyrolytic carbon may be disposed therebetween. To maintain the dense carbide shell in compression during the fuel particle lifetime, an outer isotropic carbon shell is provided which shrinks down onto the carbide shell as a result of high temperature, fast neutron irradiation. To prevent stresses of too great a magnitude from being created early in the fuel particle lifetimes, an intermediate layer of compressible pyrolytic carbon is disposed immediately exterior of the carbide shell. Nuclear reactors utilizing such particles can be operated at temperatures about 800* C. to a fast neutron dosage over 4 X 1021 neutrons/cm.2 while maintaining good fission product retentivity within the coated fuel particles.

United States Patent Goeddel 1 Mar. 21, 1972 [54] NUCLEAR FUEL PARTICLES[72] Inventor: Walter V. Goeddel, Poway, Calif.

[73] Assignee: The United States of America as represented by the UnitedStates Atomic Energy Commission [22] Filed: Oct. 9, 1969 [21] Appl.No.:865,017

[52] US. Cl ..176/68, 176/91 SP [51] Int. Cl ..G2lc 3/06 [58] FieldofSearch ..176/67, 68, 91, 91 SP [56] References Cited UNITED STATESPATENTS 3,166,614 1/1965 Taylor ..176/91 X 3,179,723 4/1965 Goeddel..176/68 X 3,212,989 10/1965 Fitzer et al..... .....176/71 3,249,5095/1966 Blocher et a1 ..176/67 3,290,223 12/1966 Blocher et al ..176/673.298.921 1/1967 Bokros et al ..176/67 3,312,597 4/1967 Glueckauf..176/67 3,325,363 6/1967 Goeddel ..176/67 Goeddel ..176/91 X Bokros etal ..176/67 [57] ABSTRACT Nuclear fuel particles having fissile orfertile central cores surrounded by a buffer layer of low densitypyrolytic carbon. A shell of dense silicon carbide, zirconium carbide orniobium carbide surrounds the buffer layer, and a layer of isotropicpyrolytic'carbon may be disposed the'rebetween. To maintain the densecarbide shell in compression during the fuel particle lifetime, an outerisotropic carbon shell is provided which shrinks down onto the carbideshell as a result of high temperature, fast neutron irradiation. Toprevent stresses of too great a magnitude from being created early inthe fuel particle lifetimes, an intermediate layer of compressiblepyrolytic carbon is disposed immediately exterior of the carbide shell.Nuclear reactors utilizing such particles can be operated attemperatures about 800 C. to a fast neutron dosage over 4 X 10neutrons/cm. while maintaining good fission product retentivity withinthe coated fuel particles.

8 Claims, 3 Drawing Figures PATENTEUMAR21 I972 3,650, 896

m a @lo n ado solo aim 4 a @10 NUCLEAR FUEL PARTICLES This inventionrelates to fuel particles for nuclear reactors and, more particularly,to fuel particles having coatings of pyrolytic carbon and siliconcarbide and to methods for operating nuclear reactors for prolongedperiods using such particles under conditions of high temperature andhigh level fast neutron irradiation.

It is well known that pyrolytic carbon coatings are useful in protectingparticles of nuclear fuel, i.e., fissile and/or fertile materials, suchas uranium, plutonium and thorium and suitable compounds thereof. Thereare advantages to be gained if fuel particles coatings have sufficientimpermeability to retain gaseous and metallic .fission products withinthe confines thereof. To meet this objective throughout the life of thenuclear fuel particles, the coatings should maintain their structuralintegrity although exposed to high temperatures and irradiation overprolonged periods of reactor operation. Examples of fuel particlesemploying pyrolytic carbon coatings are disclosed and described in US.Pat. No. 3,335,063, issued I Aug. 8, 1967, in the names of Walter V.Goeddel, Charles S.

Luby and Jack Chin; US. Pat. No. 3,298,921, issued Jan. 17, 1968 in thenames ofJack C. Bokros, Walter V. Goeddel, Jack Chin and Robert J.Price; and US Pat. No. 3,361,638, issued Jan. 2, 1968, in the names ofJack C. Bokros and Alan S. Schwarz. Although these fuel particles arewell suited for many nuclear energy applications, nuclear fuel particleshaving still better fission product retention characteristics are alwaysdesired.

It is an object of the present invention to provide improved coated fuelparticles which include a dense carbide layer. A further object is toprovide coated nuclear fuel particles which will exhibit excellentfission product retention, although subjected to high temperatures and ahigh level of fast neutron irradiation for a prolonged period of time.Still another object is to provide a method for operating a nuclearreactor at high temperatures under conditions of fast neutronirradiation wherein the fuel is employed for a relatively long lifetime.

These and other objects of the invention should be apparent from thefollowing detailed description when read in conjunction with theaccompanying drawings wherein:

FIG. 1 is a cross-sectional view ofa nuclear fuel particle embodyingvarious features of the invention;

FIG. 2 is a graphical representation of the stress versus neutron dosagefor various of the coating layers of a nuclear fuel particle deviatingslightly from that shown in FIG. 1; and

FIG. 3 is a graphical representation similar to that of FIG. 2 for thenuclear fuel particle illustrated in FIG. 1.

The integrity ofa dense carbide shell in a nuclear fuel particle coatingcan be substantially enhanced if the shell is maintained in compression.An outer isotropic pyrolytic carbon shell is considered well-suited forapplying a compressive stress to the silicon carbide shell because sucha pyrolytic carbon can be deposited with crystalline characteristicsthat will cause it to shrink at a desired rate onto the underlyingsilicon carbide shell under exposure to high temperatures and high levelneutron irradiation. Isotropic pyrolytic carbon in the density range of1.5 to about 1.7 grams per cm. exhibits a desirable rate of shrinkageduring later stages of irradiation where it is quite important tocounterbalance the interior pressure that gradually accumulates withinthe carbide shell from the creation of gaseous fission products.However, such isotropic pyrolytic carbon shrinks at a fairly high rateduring the early stages of irradiation, and stresses might be createdeither in the underlying silicon carbide layer or in the pyrolyticcarbon layer itself during this period which might later adverselyaffect the integrity thereof.

It has been found that by providing an intermediate layer ofcompressible pyrolytic carbon between the outer surface of the densecarbide shell and the inner surface of the isotropic pyrolytic carbonshell that this problem during the early stages of irradiation iseliminated. It is found that such an intermediate pyrolytic carbon layeris compressed during the early stages of irradiation and becomessufficiently compact to subsequently transmit the compressive force fromthe shrinking isotropic carbon shell inward to the dense carbide shellprior to the time that the gas pressure within the dense carbide shellreaches a substantial magnitude.

Shown in FIG. 1 is a fuel particle 10 embodying various features of theinvention. The fuel particle 10 includes a central core 12 of fissile orfertile material which may have any suita ble shape. However, forparticles of this type the core is usually not greater than a millimeterin size. Nuclear fuel in the form of spheriods between about microns and500 microns in diameter are preferred for many applications, althoughlarger and smaller spheriods may be used. The core materials may be incarbide form or in other suitable forms. such as the oxide, nitride orsilicide, which are stable at relatively high temperatures. The termnuclear fuel is used in its usual sense to include the fissionable orfertile isotopes of uranium, thorium and plutonium.

Nuclear fuel materials have a tendency to expand during high temperatureoperations, and upon fissioning gaseous and metallic fission productsare created Accommodation of these effects is important in providing afuel particle suitable for prolonged operation under conditions of highneutron flux; and particularly if a dense nuclear fuel core 12 isemployed, a layer 13 of low density buffer material is desirablydisposed near the outer surface of the core. The buffer material should,of course, be compatible with the core material both in the environmentin which it is initially deposited and in the environment where in thefuel particle 10 is employed. It has been found that low density (notgreater than about 60 percent of theoretical maximum density) pyrolyticcarbon is preferable for use with nuclear fuels.

Spongy pyrolytic carbon is preferred as a buffer material and is definedas soot-like amorphous carbon having a dif fused X-ray diffractionpattern and a density of about 13 grams per cm. or below. Spongy carbonis considered to excellently attenuate fision recoils, and it therebyprevents structural damage to outer layers of the fuel particle 10 whichwill provide the fission-product retentive pressure vessel. To servethese various purposes, spongy carbon is usually employed at a thicknessof at least about 20 microns and layers of up to about 100 microns mightbe employed. Generally, spongy carbon in the range ofabout 50 to 60microns is used.

It may be desirable in fuel particle production operations to provide athin seal layer 15 immediately exterior of the buffer layer 13,particularly when oxide core materials are used. The seal layer 15 isgenerally as thin as possible commensurate with the desired function ofproviding a gas barrier during the remainder of the production stepswherein the outer layers which form the pressuretight jacket aredeposited. Dense laminar or isotropic pyrolytic carbon is consideredsuitable for the seal layer 15, and dense laminar carbon is preferred.Generally, laminar carbon having a density of at least about 1.7 gramsper cm. will provide a sufficient gas barrier at a thickness as low as 1micron. Because it is often difficult to determine whether extremelysmall particles, such as those having diameters between 100 and 500microns, are uniformly coated, a seal layer 15 between about 2 to 5microns is usually preferred when oxide core materials are used. Thespongy carbon buffer layer 13 is somewhat fragile, and the seal layer 15also provides some mechanical protection to the buffer layer 13 whichfacilitates handling for the inspection purposes or for transfer toanother coating apparatus for deposition of the outer layers. To takefull advantage of this feature, the seal layer 15 is deposited in thesame coating apparatus wherein the buffer layer 13 is applied.

Although various high temperature-stable metal or metalloid carbides canbe used for a shell 17 to provide the desired fission-product retention,from a standpoint of neutron economy it is likely that silicon, niobiumor zirconium carbide, or a mixture thereof, would be employed because oftheir relatively low neutron capture cross sections. Silicon carbide iscommonly employed because it is relatively inexpensive and because thereis presently available a fairly large accumulation of data on thepyrolysis of methyltricholorsilane in the presence of an excess ofhydrogen gas, lone of the reactions commonly employed to deposit siliconcarbide from the vapor phase. The seal layer or an inner layer 21 ofisotropic pyrolytic carbon, described hereinafter, provides a gaseousbarrier which eliminates any potential reaction between hydrogenchloride gas (a byproduct of the aforedescribed pyrolysis reaction) withuranium that may be present in the central core 12. Generally, thecarbide shell 17 is employed at a thickness between about and 30 micronsto provide the desired assistance in the containment of fission productsand to provide a structural body having sufficient strength to maintainits integrity both during handling for subsequent coating operations andthereafter during its lifetime in a nuclear reactor. Although siliconcarbide shells of greater or slightly lesser thickness can be employed,it seems unlikely from a standpoint of neutron economy that a layergreater than about 40 microns in thickness would be used. Generally, thecarbide is employed in the thickness ranges above-enumerated having adensity of at least about 90 percent ofits theoretical maximum density.

In order to prevent fracture of the dense carbide shell 17 andconsequent failure of the coated fuel particle 10 as a fission-productretentive pressure vessel, the structural stress within the carbideshell 17 should be maintained within a particular range throughout thelife of the fuel particle. As previously indicated. an isotropicpyrolytic carbon shell 19 is disposed exterior of the carbide shell 17having the crystalline characteristic that it undergoes controlledshrinkage under neutron irradiation. The shrinkage imposes a compressiveforce on the dense carbide shell 17 which counteracts the internalforces resulting from the buildup of gaseous fission productstherewithin. This balance becomes more important as the lifetime of thefuel particle 10 within a nuclear reactor core increases and theinternal pressure of gaseous fission products reaches higher and higherlevels.

It may also be desirable to employ a dense isotropic pyrolytic carbonlayer 21 interior of the silicon carbide layer 17 to prevent possiblechemical reaction between the materials in the nuclear fuel core 12 andthe silicon carbide as a result of thermally induced migration whichmight occur during the lifetime of the fuel particle 10. It may bepossible for metal atoms from the core 12 to migrate outward through thebuffer layer 13 under high neutron irradiation conditions. Suchmigration might result in uranium and fission products from the corechemically reacting with the silicon carbide, However, the provision ofan adequate layer 21 of isotropic pyrolytic carbon interior of thecarbide shell substantially eliminates such a potential interactionthroughout the anticipated lifetime of the fuel particles 10.

To achieve the desired function, the isotropic carbon employed in thelayer 21 should usually be between about 15 and microns thick and have adensity in the range of about 1.7 to 1.95 grams per cm.. The isotropy ofcarbon is conveniently measured in terms of its Bacon Anisotropy Factor(BAF). The technique of measurement and a complete explanation of thescale of measurement is set forth in an article by G. E. Bacon entitledA Method for Determining the Degree of Orientation of Graphite whichappeared in the Journal of Applied Chemistry, Volume 6, page 477 (1956).Under the Bacon system of measurement, perfectly isotropic carbon has aBAF of L0. The pyrolytic carbon employed for the layer 21 shouldpreferably have a BAF of about 1.2 or less.

The outer pyrolytic carbon shell 19 is preferably impermeable to gas andof such a character that it maintains the metal carbide shell 17 in astate of slight compression throughout continuing neutron irradiation inspite of the continuing pressure buildup therewithin. The exterior shell19 thus structurally reinforces the silicon carbide and provides asecondary gas barrier while also protecting the carbide shell 17 fromdamage from without. However, as previously indicated, thecharacteristics of the outermost shell 19 are chosen so as to exert thedesired compressive force on the interior carbide shell 17.

It has been found that isotropic pyrolytic carbon having a densitybetween about 1.5 and about 1.7 grams per cm. exhibits the desiredcontrolled shrinkage in a direction parallel to the plane of depositionto maintain the state of compression desired in the carbide shell duringthe later stages of neutron irradiation. The isotropic carbon shellshould also be at least about 20 microns thick, and it is likely that anisotropic carbon shell between about 20 and 40 microns thick would beused. Preferably, the isotropic pyrolytic carbon employed has a BAF ofabout 1.2 or less, and an average crystallite size of between about 35and A.

The layer 23 appearing intermediate the exterior surface of the carbideshell 17 and the interior surface of the outer pyrolytic carbon shell isthe compressible pyrolytic carbon layer referred to earlier. The detailsof the compressible layer 23 are set forth hereinafter,

FIG. 2 is a graphical representation wherein stress, measured in p.s.i.,is plotted against total neutron dosage, measured as N/cm. and which isexpressed as a multiple of neutron flux and time. For purposes of thisapplication, neutron flux is measured as neutrons per square centimeterper second, using fast neutrons having an energy greater than about 0.18Mev.

FIG. 2 is representative of a profile of the stress in the carbide shell17 and in the exterior isotropic pyrolytic carbon shell 19 for fuelparticles generally similar to the fuel particle 10 shown in FIG. 1wherein no intermediate layer 23 is provided and the isotropic pyrolyticcarbon shell 19 is deposited directly upon the silicon carbide shell 17.In FIGS. 2 and 3, the region above the abscissa indicates tensile stresswhereas the area therebelow indicates compressive stress. The uppercurve marked A in FIG. 2 is a plot of the tangential tensile stress atthe inner surface of the isotropic pyrolytic carbon outer shell 19, andthe lower curve marked B is a plot of the tangential compressive stressin the silicon carbide shell. As is apparent from the curves, during theinitial stages ofirradiation, i.e., up to a dosage of about 2 X 10 N/cm.(E 0.l8 Mev), shrinkage or densification in the pyrolytic carbon occursat a fairly rapid rate causing the silicon carbide shell 17 to be placedunder a fairly high compressive stress while a counteracting tensilestress of nearly similar magnitude is created in the isotropic pyrolyticcarbon shell 19. As the neutron irradiation dosage continues toincrease, it is seen that the tensile stress curve begins to decrease asa result of stress relaxation due to irradiation induced creep in thepyrolytic carbon and as a result ofa reduced rate of shrinkage in thepyrolytic carbon.

After a level of about 2-3 X 10 N/cm. is reached, it can be seen thatthe curve A substantially levels off. In this region of neutron dosage,the shrinkage experienced by the isotropic pyrolytic carbon shell 19 ina direction parallel to the plane of deposition is at a rate about equalto balance the stresses due to the rate of increase in fission productpressure interior of the silicon carbideshell 17 so that the net resultis that the tensile stress within the isotropic carbon shell and thecompressive stress in the carbide shell remain at about the same values.

The isotropic pyrolytic carbon shell 19 may not be able to withstand therate of buildup of tensile stress which occurs during the early neutronirradiation as a result of the very rapid initial densificationphenomena, and cracking on a microscale may occur which may ultimatelyresult in the subsequent failure of the isotropic pyrolytic carbonshell. However, it has been found that if an intermediate pyrolyticcarbon layer 23 is disposed between the carbide shell 17 and theexterior isotropic pyrolytic carbon shell 19, the stress curves for theisotropic pyrolytic carbon and the carbide shell straighten out toacceptable and desirable slopes. The intermediate pyrolytic carbon layer23 is chosen with crystalline characteristics and a predetermineddensity and thickness so that it is compressed during the initialirradiation dosage period and thus provides space into which theisotropic pyrolytic carbon shell 19 can shrink while transferring verylittle inward force to the carbide shell 17. However, thecharacteristics of the intermediate layer 23 are chosen such that it hassubstantially completed its compression by the time that a neutrondosage equal to 2-3 X N/cm (E B0. l 8 Mev) is reached.

FIG. 3 is a graphical representation similar to that shown in FlG. 2 inwhich curve A represents the tensile stress in the isotropic pyrolyticcarbon shell 19 in a fuel particle 10 wherein such an intermediate layer23 of compressible pyrolytic carbon is employed. B is a curve indicatingthe compressive stress created in the carbide shell 17. As can be seen,both the tensile stress in the exterior isotropic pyrolytic car bonshell 19 and the compressive stress in the silicon carbide shell 17develop at substantially even rate. In order to provide the desiredamount of compressibility, it has been found that the intermediate layer23 should have a density of between about 07 and about 1.2 grams percmi. The thickness of the intermediate pyrolytic carbon layer employedvaries between 10 and 30 microns. Generally, when carbon having adensity of about 1.2 grams per cm. (near the upper end of the de nsityrange) is used, a thickness of near 30 microns is employed. Oppositely,when carbon having a density near the lower end ofthe density range,i.e., about 0.7 grams per cm, is used, the thickness generally employedwill be closer to 10 microns.

The pyrolytic carbon employed for the intermediate layer 23 should besimilar in structure to the spongy buffer layer 13. Regardless of theprecise crystalline structure of the pyrolytic carbon employed, carbonwithin this density range should exhibit the desired compressibilityduring the initial neutron irradiation period, thus delaying anysignificant transfer of force to the interior carbide shell 17.Preferably, the intermediate layer is made of spongy pyrolytic carbonhaving an average crystallite size (Lc) below about 50 A. for goodperformance throughout the density range.

High temperature gas-cooled nuclear reactors, such as the HTGR reactornow operating at Peach Bottom, Pennsylvania, can be efficiently operatedover relatively long fuel lifetimes using particles made in accordancewith the foregoing principles Such coated fuel particles are designed toexhibit excellent fission retentivity although exposed to temperaturesbetween about 800 and 1350C. for neutron dosages up to as high as 8-9 X10 N/cm. (E 0.l8 Mev.) within the core of such a nuclear reactor.

The following example illustrates a process for producing the coatednuclear fuel particles having various features of the invention. Itshould be understood of course that this example is only illustrativeand does not constitute limitations upon the scope of the invention.

EXAMPLE Particulate thorium-uranium dicarbide is prepared having aparticle size of about 200 microns and being generally spheroidal inshape. The material used has a thorium to uranium mole ratio of about 3to l. A graphite reaction tube having an internal diameter ofabout 2.5inches is heated to about 1,100 C. while a flow of helium is maintainedtherethrough. When the coating is ready to begin, a charge of 100 gramsof the cores of thorium-uranium dicarbide is fed into the top of thereaction tube while the flow of helium upward through the tube ismaintained at a rate sufficient to levitate the cores and create withinthe tube a fluidized particle bed.

When the temperature ofthe cores reaches about l,lOOC., acetylene gas isadmixed with helium to provide an upwardly flowing gas stream having atotal flow rate about 10,000 em /min. and a partial pressure ofacetylene of about 0.8 (total pressure of 1 atmosphere), The acetylenedecomposes and deposits low density, spongy carbon upon the cores. Flowof the acetylene is continued for a sufficient time to deposit upon eachof the cores a layer of about 50 microns thick of spongy pyrolyticcarbon having a density of about 1.0 gram/emf.

The flow of acetylene then terminated, and the temperature is raised toabout 2,000 C. When this temperature is reached a gas flow of about2,000 cm /min. of methane and 8,000 cm."/min. of helium is employed.After about minutes, the

methane flow is terminated. By this time, a layer approximately 20microns thick of isotropic pyrolytic carbon having a density of about1.9 grams/cm. and a BAF of about 1.05 is deposited on each of theparticles.

The temperature is then lowered to about [500 C. Hydrogen is employed asthe fluidizing gas at a rate of about 10,000 cm. /min., and about 10percent of the hydrogen stream is bubbled through a bath ofmethyltrichlorosilate at room temperature. These conditions aremaintained for about 1 hour, by the end of which time silicon carbide isuniformly deposited on each of the carbon-coated spheroids in the formof a layer about 20 microns thick. Subsequent examination andmeasurement shows that the silicon carbide is beta phase silicon carbidehaving a density of about 3.19 grams/cm, which is about 99 percent ofthe theoretical density of silicon carbide (3.215 g./cm.).

The silicon-carbide coated cores are maintained in fluidized 7condition, substituting helium as the fluidizing gas. and thetemperature is adjusted to about 1,000C. At this temperature the flow ofgas is altered to provide a mixture of acetylene at 9,000 cm. /min. andhelium at 1,000 cmP/min. Coating is carried on for about 3 minutes,during which time a layer about 20 microns thick of spongy pyrolyticcarbon is deposited upon each of the silicon carbide-coated spheroids.The carbon has a density of about 1.1 grams/cm. and an averagecrystallite size of about 20 A.

The temperature is raised to about l,400 C., and the flow of gas isaltered to provide a mixture of propane at 4,000 cm. /min., and heliumat 6,000 cmflmin. Coating is carried on for about 5 minutes, duringwhich time a layer of about 50 microns thick of isotropic carbon isdeposited on each of the coated spheroids. The coated spheroids areslowly cooled at room temperature and examined. The isotropic carbon hasa density of about 1.9 grams/cm. and a BAF of about 1.04.

Testing of the coated particles is carried out by disposing them in asuitable capsule and subjecting them to neutron ir radiation at anaverage temperature of about 1,3 25 C. During the time of irradiation,the total fast neutron dose is estimated to be about 8 X 10 neutrons/cm.using neutrons of energy greater than about 0.18 Mev. During thisperiod, burnup of over 20 percent of the metal atoms in the nuclear fuelcores occurs. No coating failures are apparent, and the dimensionalstability of the coatings is considered completely satisfactory. Thefission-product retention of the particles is well within acceptablelimits, and these particles are considered to be well suited for use ina high temperature gas-cooled power reactor.

The foregoing process is repeated except that the intermediate layer ofpyrolytic carbon is omitted and the 50 micron thick outer layer ofisotropic pyrolytic carbon is deposited directly upon thesilicon-carbide shell. Testing of these particles is carried out in thesame manner as described above to an irradiation dosage of 4 X 10neutrons/cm (E 0.l8 Mev.) Examination of the particles shows asignificant number of coating failures, and it is considered that thesecoating failures result from the creation of stresses in the isotropicpyrolytic carbon shell during the initial irradiation when densificationoccurs at a fairly rapid rate within the isotropic pyrolytic carbonshell.

Various features of the invention are set forth in the claims thatfollow.

What is claimed is:

1. A nuclear fuel particle comprising a central core of fissile orfertile material, a layer of low density buffer material surroundingsaid core, a dense carbide shell exterior of said buffer layer made of amaterial selected from the group consisting of silicon carbide,zirconium carbide, niobium carbide and mixtures thereof, a shell ofisotropic pyrolytic carbon having a density of between about 1.5 g./cm.and about 1.7 g./cm. exterior of said carbide shell, and an intermediatelayer ofspongy pyrolytic carbon disposed between and in contact withsaid carbide shell and said isotropic pyrolytic carbon shell and havinga density between about 0.7 and about 1.2 g./cm.".

6. A nuclear fuel particle in accordance with claim 5 wherein saidisotropic carbon has a Bacon Anisotropy Factor between 1.0 and about L2.

7. A nuclear fuel particle in accordance with claim 1 wherein anisotropic carbon layer is disposed intcriorly of said carbide shell.

8.- A nuclear fuel particle in accordance with claim 7 wherein saidinterior isotropic carbon layer is at least about 15 microns thick andhas a density between about 1.7 g./cm. and about 1.95 g./cm.

2. A nuclear fuel particle in accordance with claim 1 wherein saidintermediate layer is between about 10 and about 30 microns thick.
 3. Anuclear fuel particle in accordance with claim 2 wherein said carbideshell is made of silicon carbide having a density between about 3.17 andabout 3.20 g./cm.3.
 4. A nuclear fuel particle in accordance with claim1 wherein said carbide shell is at least about 20 microns thick.
 5. Anuclear fuel particle in accordance with claim 1 wherein said isotropiccarbon shell is at least about 20 microns thick.
 6. A nuclear fuelparticle in accordance with claim 5 wherein said isotropic carbon has aBacon Anisotropy Factor between 1.0 and about 1.2.
 7. A nuclear fuelparticle in accordance with claim 1 wherein an isotropic carbon layer isdisposed interiorly of said carbide shell.
 8. A nuclear fuel particle inaccordance with claim 7 wherein said interior isotropic carbon layer isat least about 15 microns thick and has a density between about 1.7g./cm.3 and about 1.95 g./cm.3.